The present invention relates to a noise reduction system for a passenger compartment of an automotive vehicle by positively generating the sound to cancel the vehicle internal noise.
There have been proposed several techniques for reducing the bass sound generated mainly from an engine and transmitted to the passenger compartment by producing canceling sound from an additional sound source such as a speaker disposed in the passenger compartment. The amplitude of the canceling sound is the same as that of the bass sound, but the canceling sound has a reversed phase to that of bass sound.
For example, Japanese patent application laid open No. 1988-315346 discloses a noise reduction technique for reducing the bass sound by detecting the engine speed from the output interval of the ignition signal, retrieving the canceling sound determined at every engine speed range, outputting the canceling sound from a speaker, on the other hand, receiving the bass sound in the passenger compartment from a microphone disposed at a noise receiving position, comparing the bass sound with the previous bass sound, in case of a low (high) level of the inputted bass sound outputting the canceling sound with an advanced (retarded) phase or with a lowered (heightened) amplitude from the speaker, and controlling the canceling sound so as to minimize the input level of the bass sound received from the microphone.
In this prior art technology, however, since the engine speed fluctuates incessantly during the vehicle traveling, and fluctuates violently particularly during a transient engine operation, even if an optimum canceling sound is outputted for each engine speed range, the output waveform of the canceling sound signal becomes discontinuous, so that an uncomfortable noise is produced when the connection timing is put out of order.
To overcome this problem, for example, Japanese patent application laid open No. 1991-90448 proposes a technique to prevent the uncomfortable noise from being generated by providing a so-called waiting time during which the canceling sound is not outputted, whereby a smooth connection of the canceling sound can be made before and after a fluctuation of the engine speed.
However, in this prior art technique for reducing bass sound, when the vehicle is started, since the bass sound can not be reduced positively during a transient engine operation, the bass sound caused by an engine is transmitted directly into the passenger compartment. Also when the vehicle is transferred to a constant speed traveling, the bass sound is substantially eliminated by the canceling sound. In these ways, since sometime the bass sound is eliminated and sometime it is produced, consequently a vehicle driver or a passenger becomes uncomfortable.